Biodiesel generally is a fuel, often employed as a petroleum diesel replacement fuel made from fats, oils, and greases. Biodiesel is technically defined as a fuel that comprises mono-alkyl esters of FFAs derived from vegetable oils or animal fats and which meets the requirements of ASTM D 6751, the entire disclosure of which is incorporated herein by reference.
Conventional biodiesel production generally uses, as a feedback, oils and/or fats that are primarily composed of triglycerides (TGs) and may further contain small amounts of FFAs.
Fats and oils are generally insoluble in water but soluble in many organic solvents. They generally have lower densities than water, and may have consistencies at ambient temperature (about 25° C.) of solid, semi-solid, or clear liquid. When they are solid, appearing at room temperature, they are referred to as “fats,” and when they are liquid at that temperature, they are called “oils.” For simplification, the term “oil” is generally used herein to refer to both fats and oils that originate from animal or vegetable sources (as opposed to petroleum-based oils).
Oils are classified as “lipids” which is a category that embraces a broad group of chemical substances. In addition to TGs, the term “lipids” also includes monoglycerides (MGs), diglycerides (DGs), FFAs and other substances, including, but not limited to, fatty alcohols and tocopherols. Other than the FFAs, crude vegetable oils generally contain up to about two percent total of these minor components. Animal fats generally contain smaller amounts of these other substances than do vegetable oils.
Structurally, the TGs which make up fats and oils generally consist of three fatty acids attached to one glycerol molecule. If all three fatty acids are identical, the TG is generally referred to as a “simple” TG. The more common forms, however, are “mixed” TGs in which two, three, or more different fatty acids are present in the fatty acid. The general chemical structure of TGs is shown in Scheme 1 below, wherein structure IA illustrates a simple TG and structure IB illustrates a mixed TG. The R groups on the TGs are typically linear hydrocarbon chains comprising up to about 21 carbons and optionally up to about 6 double bonds.

Oils may also include partial degradation products derived from TGs, such as MGs, exemplified by general formula IIA and IIB in Scheme 2 below, and DGs, exemplified by general formulae IIC and IID in Scheme 2.

Conversion of oils to alkyl esters of FFAs (including, but not limited to, biodiesel) is typically accomplished by a process of transesterification, wherein the glycerol portion of a glyceride, e.g., a TG, is replaced with an alcohol, generally methanol or ethanol. The reaction is generally base catalyzed and carried out at atmosphere pressure. Conventional biodiesel production processes generally do not work well for esterification/transesterification of feedback containing levels of FFAs that are higher than about 2% because FFAs and bases generally react under such conditions to form soaps. For feedback containing more than about 2% FFA, a two-step procedure is generally required, typically including an acid-catalyzed pretreatment targeting conversion of FFA to esters, followed by a base-catalyzed reaction to convert remaining glycerides to esters, both steps generally performed at atmospheric pressure.
Animal fats from rendering operations, as well as used cooking oils, including, for example, restaurant trap grease, provide a potentially attractive feedstock for biodiesel production. The expression “restaurant trap grease” generally refers to grease that has traveled down a drain at a restaurant often captured in a grease interceptor, or “trap,” before it enters a sanitary sewer, as well as grease generally produced by cooking or food preparation establishments. However, such feedstock typically contains high levels of FFAs as a result of hydrolytic degradation of TGs, generally resulting from microbial, chemical or thermal reaction. Hydrolysis of a TG molecule is depicted in Scheme 3 below.

Elevated levels of FFAs generally complicate, increase process time, and raise the cost of conversion to biodiesel.
Restaurant trap grease is a low cost waste material. It is, however, a complex and challenging feedstock from the standpoint of impurities, variability and processing requirements. It varies widely in composition, often including, in varying proportions, fats, oils, and greases as well as food particles, dirt, water and other materials that are carried down drains.
There is a need for a process capable of converting feedstocks, such as restaurant trap grease, which contain higher levels of FFAs and which may provide a cost-effective way to convert feedstock containing used oils, such as restaurant trap grease, into esters of FFAs, including, but not limited to, biodiesel.